Commit 201142d1 authored by Vitaly Kuznetsov's avatar Vitaly Kuznetsov Committed by Sean Christopherson
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KVM: selftests: Compare wall time from xen shinfo against KVM_GET_CLOCK 



xen_shinfo_test is observed to be flaky failing sporadically with
"VM time too old". With min_ts/max_ts debug print added:

Wall clock (v 3269818) 1704906491.986255664
Time info 1: v 1282712 tsc 33530585736 time 14014430025 mul 3587552223 shift 4294967295 flags 1
Time info 2: v 1282712 tsc 33530585736 time 14014430025 mul 3587552223 shift 4294967295 flags 1
min_ts: 1704906491.986312153
max_ts: 1704906506.001006963
==== Test Assertion Failure ====
  x86_64/xen_shinfo_test.c:1003: cmp_timespec(&min_ts, &vm_ts) <= 0
  pid=32724 tid=32724 errno=4 - Interrupted system call
     1	0x00000000004030ad: main at xen_shinfo_test.c:1003
     2	0x00007fca6b23feaf: ?? ??:0
     3	0x00007fca6b23ff5f: ?? ??:0
     4	0x0000000000405e04: _start at ??:?
  VM time too old

The test compares wall clock data from shinfo (which is the output of
kvm_get_wall_clock_epoch()) against clock_gettime(CLOCK_REALTIME) in the
host system before the VM is created. In the example above, it compares

 shinfo: 1704906491.986255664 vs min_ts: 1704906491.986312153

and fails as the later is greater than the former.  While this sounds like
a sane test, it doesn't pass reality check: kvm_get_wall_clock_epoch()
calculates guest's epoch (realtime when the guest was created) by
subtracting kvmclock from the current realtime and the calculation happens
when shinfo is setup. The problem is that kvmclock is a raw clock and
realtime clock is affected by NTP. This means that if realtime ticks with a
slightly reduced frequency, "guest's epoch" calculated by
kvm_get_wall_clock_epoch() will actually tick backwards! This is not a big
issue from guest's perspective as the guest can't really observe this but
this epoch can't be compared with a fixed clock_gettime() on the host.

Replace the check with comparing wall clock data from shinfo to
KVM_GET_CLOCK. The later gives both realtime and kvmclock so guest's epoch
can be calculated by subtraction. Note, CLOCK_REALTIME is susceptible to
leap seconds jumps but there's no better alternative in KVM at this
moment. Leave a comment and accept 1s delta.

Reported-by: default avatarJan Richter <jarichte@redhat.com>
Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240206151950.31174-1-vkuznets@redhat.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent d85465f2

tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c

+28 −22
Original line number Diff line number Diff line
@@ -380,20 +380,6 @@ static void guest_code(void) 
	GUEST_SYNC(TEST_DONE);

}



static int cmp_timespec(struct timespec *a, struct timespec *b)

{

	if (a->tv_sec > b->tv_sec)

		return 1;

	else if (a->tv_sec < b->tv_sec)

		return -1;

	else if (a->tv_nsec > b->tv_nsec)

		return 1;

	else if (a->tv_nsec < b->tv_nsec)

		return -1;

	else

		return 0;

}



static struct shared_info *shinfo;

static struct vcpu_info *vinfo;

static struct kvm_vcpu *vcpu;
@@ -449,7 +435,6 @@ static void *juggle_shinfo_state(void *arg) 


int main(int argc, char *argv[])

{

	struct timespec min_ts, max_ts, vm_ts;

	struct kvm_xen_hvm_attr evt_reset;

	struct kvm_vm *vm;

	pthread_t thread;
@@ -468,8 +453,6 @@ int main(int argc, char *argv[]) 
	bool do_evtchn_tests = do_eventfd_tests && !!(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_SEND);

	bool has_shinfo_hva = !!(xen_caps & KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA);



	clock_gettime(CLOCK_REALTIME, &min_ts);



	vm = vm_create_with_one_vcpu(&vcpu, guest_code);



	/* Map a region for the shared_info page */
@@ -1010,7 +993,6 @@ int main(int argc, char *argv[]) 
	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &evt_reset);



	alarm(0);

	clock_gettime(CLOCK_REALTIME, &max_ts);



	/*

	 * Just a *really* basic check that things are being put in the
@@ -1019,6 +1001,8 @@ int main(int argc, char *argv[]) 
	 */

	struct pvclock_wall_clock *wc;

	struct pvclock_vcpu_time_info *ti, *ti2;

	struct kvm_clock_data kcdata;

	long long delta;



	wc = addr_gpa2hva(vm, SHINFO_REGION_GPA + 0xc00);

	ti = addr_gpa2hva(vm, SHINFO_REGION_GPA + 0x40 + 0x20);
@@ -1034,12 +1018,34 @@ int main(int argc, char *argv[]) 
		       ti2->tsc_shift, ti2->flags);

	}



	vm_ts.tv_sec = wc->sec;

	vm_ts.tv_nsec = wc->nsec;

	TEST_ASSERT(wc->version && !(wc->version & 1),

		    "Bad wallclock version %x", wc->version);

	TEST_ASSERT(cmp_timespec(&min_ts, &vm_ts) <= 0, "VM time too old");

	TEST_ASSERT(cmp_timespec(&max_ts, &vm_ts) >= 0, "VM time too new");



	vm_ioctl(vm, KVM_GET_CLOCK, &kcdata);



	if (kcdata.flags & KVM_CLOCK_REALTIME) {

		if (verbose) {

			printf("KVM_GET_CLOCK clock: %lld.%09lld\n",

			       kcdata.clock / NSEC_PER_SEC, kcdata.clock % NSEC_PER_SEC);

			printf("KVM_GET_CLOCK realtime: %lld.%09lld\n",

			       kcdata.realtime / NSEC_PER_SEC, kcdata.realtime % NSEC_PER_SEC);

		}



		delta = (wc->sec * NSEC_PER_SEC + wc->nsec) - (kcdata.realtime - kcdata.clock);



		/*

		 * KVM_GET_CLOCK gives CLOCK_REALTIME which jumps on leap seconds updates but

		 * unfortunately KVM doesn't currently offer a CLOCK_TAI alternative. Accept 1s

		 * delta as testing clock accuracy is not the goal here. The test just needs to

		 * check that the value in shinfo is somewhat sane.

		 */

		TEST_ASSERT(llabs(delta) < NSEC_PER_SEC,

			    "Guest's epoch from shinfo %d.%09d differs from KVM_GET_CLOCK %lld.%lld",

			    wc->sec, wc->nsec, (kcdata.realtime - kcdata.clock) / NSEC_PER_SEC,

			    (kcdata.realtime - kcdata.clock) % NSEC_PER_SEC);

	} else {

		pr_info("Missing KVM_CLOCK_REALTIME, skipping shinfo epoch sanity check\n");

	}



	TEST_ASSERT(ti->version && !(ti->version & 1),

		    "Bad time_info version %x", ti->version);